Enterprise networks are typically organized in clusters of smaller networks or sub-networks for ease of management and scalability. User clients that connect to these sub-networks usually function in a plug-and-play mode where these clients fetch required infrastructure information for network connectivity from dedicated servers in the sub-network the clients are connected to. The clients usually use an interne protocol to identify such dedicated servers and obtain the required infrastructure information. Examples of such protocols include Dynamic Host Configuration Protocol (DHCP) and Time Protocol (TIMEP). In such protocols, the client uses a broadcast discovery mechanism to identify the availability of the dedicated servers in the sub-network.
However, it may not be operationally feasible to dedicate at least one server in each sub-network for the purpose of providing infrastructure information for each clients plugged in to the sub-network. Therefore, such servers usually reside in one particular sub-network, and the routers of other sub-networks employ a technique of application broadcast forwarding (or commonly known as UDP broadcast forwarding) to relay broadcast discovery packets generated by clients in one sub-network to an appropriate dedicated server that resides in another sub-network.
The router in the sub-network that provides the application broadcast forwarding functionality normally also serves as the default gateway for the sub-network to other sub-networks. Hence, if this router fails, the sub-network will be completely cut-off from the entire network. Redundancy protocol is usually used to prevent such network outage caused by a single router. Examples of a redundancy protocol include Virtual Router Redundancy Protocol (VRRP) and Cisco's Hot Standby Routing Protocol (HSRP) where a pair (or more) of routers is used to provide default gateway functionality, so that outage of the sub-network will not occur in the event when one of the routers fails.
When redundancy protocol is used to provide default gateway functionality in the sub-network, some kind of arbitration mechanism is used to ensure that only one router forwards packets for that sub-network. For example, VRRP ensures that only one router respond to Address Resolution Protocol (ARP) requests for a default gateway network address, thus ensuring only one router that will forward requests and hence prevents any possibility of data duplication in the network. As there are multiple routers in the sub-network, different hosts may also be configured with different routers as their default gateway for unicast load balancing purposes.
However when such routers also provide the application broadcast forwarding functionality, there is no arbitration mechanism to dynamically decide which of the routers in the sub-network will relay broadcast packets. Therefore, all the routers will relay the broadcast packets to the sub-network where the dedicated server resides. This results in duplication of packets in the network. The server will also respond to these duplicated packets and send its replies back to the hosts in the sub-network. This results in bandwidth wastage and other operational problems in the network.
A common solution to this is to have only one router of a redundant router pair enabled with the application broadcast forwarding functionality, so that only this router will relay broadcast packets. However the failure of this router will result in clients not being able to discover the dedicated servers despite the availability of other redundant routers in the sub-network.